c++: init-list opt and lvalue initializers [PR118673]

c++: init-list opt and lvalue initializers [PR118673]

When fn returns {extension}, the ArrayRef in the initializer_list is
constructed to point to 'extension', the variable with static storage
duration.  The optimization was copying extension's value into a temporary
array and constructing the ArrayRef to point to that temporary copy instead,
resulting in a dangling pointer.  So suppress this optimization if the
element constructor takes a reference and the initializer is a non-mergeable
lvalue.

	PR c++/118673

gcc/cp/ChangeLog:

	* call.cc (maybe_init_list_as_array): Check for lvalue
	initializers.
	* cp-tree.h (enum cp_lvalue_kind_flags): Add clk_mergeable.
	* tree.cc (lvalue_kind): Return it.
	(non_mergeable_glvalue_p): New.
	(test_lvalue_kind): Adjust.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/initlist-opt6.C: New test.